A chemical exchanged saturation transfer (CEST) method utilizes a train of high-power radiofrequency (RF) saturation pulses that are applied at the resonance frequency of exchangeable protons, after which saturation is transferred through chemical exchange to a bulk water pool leading to MR signal loss that yields contrast. Known systems employ the CEST method to image tissue pH, to assess glycosaminoglycan concentration in cartilage, to map brain protein through —NH chemical residues and to monitor glycogen concentration in the liver.
Saturation may be achieved by applying a continuous RF pulse with long duration. Increasing the length of the RF pulse typically increases the CEST effect. However, technical constraints, such as limited pulse width and duty-cycle limit its application on MR clinical scanners. Pulsed train RF saturation may be used, but the available duty cycle limits the achievable CEST effect. In order to analyze the CEST effect, multiple MR images are acquired with saturation pulses at different resonance frequencies that include the specific resonance frequency associated with the solute of interest. A measurement for the saturation effect is the ratio between the signals of saturated water (S) and signal of water without saturation (S0). CEST contrast may be quantified by calculating its asymmetry in magnetization transfer ratio (MTRasym) at the frequency of the exchangeable protons (Δω)): MTRasym=(S(−Δω))−S(+Δω))/S0. The necessity to acquire multiple images typically renders CEST imaging time-consuming. Furthermore due to a long acquisition time, in vivo acquisition is susceptible to motion artifacts. Most known system CEST imaging is conducted on a high-field (>=3T) MRI scanner, since at high field the larger chemical shift dispersion permits a clearer delineation of the CEST effect and the prolonged T1 of water protons leads to a longer lifetime of saturated spins after saturation transfer from a CEST agent to bulk water. However it exacerbates the CEST sensitivity to radiofrequency field (B1) inhomogeneities. Known CEST imaging methods are burdened by prohibitively long scan times. A system according to invention principles addresses this deficiency and related problems.